Method for cross-linking polyacrylates

ABSTRACT

Method for producing a polyacrylate adhesive compound by a hot-melt method in which a polyfunctional α-cleaving initiator, which is present as an oligomer, is added to the polymer to be crosslinked prior to the hot-melt method and ultraviolet crosslinking is carried out after treatment in the hot-melt method.

This is a 371 of PCT/EP01/09555 filed 18 Aug. 2001 (international filingdate).

The invention relates to the crosslinking of pressure sensitivelyadhesive polyacrylate hotmelts with di- or polyfunctional epoxidesand/or with di- or polyfunctional alcohols.

BACKGROUND OF THE INVENTION

In the field of pressure sensitive adhesive (PSA) compositions, ongoingtechnological developments in the coating technique mean that there is aprogressive need for new developments. Within the industry, hotmeltprocesses with solventless coating technology are of increasingimportance in the preparation of PSA compositions, since theenvironmental regulations are becoming ever greater and the prices ofsolvents continue to rise. The consequence of this is that solvents areto be eliminated as far as possible from the manufacturing process forPSA tapes. The associated introduction of the hotmelt technology isimposing ever greater requirements on the adhesive compositions. AcrylicPSA compositions in particular are the subject of very intensiveinvestigations aimed at improvements. For high-level industrialapplications, polyacrylates are preferred on account of theirtransparency and weathering stability. In addition to these advantages,however, these acrylic PSA compositions may also meet stringentrequirements in respect of shear strength and bond strength. Thisprofile of requirements is met by polyacrylates of high molecular weightand high polarity with subsequent efficient crosslinking. For thecrosslinking there are in principle two methods available, since thethermal crosslinking of acrylic hotmelt PSAs can be realized only bycircuitous routes. Electron beam crosslinking offers the advantage thatcrosslinking can still be carried out at relatively high adhesiveapplication rates. The disadvantage is the inefficient crosslinking,since there are no reactive groups, such as double bonds, for example.Consequently, the quality of electron beam crosslinking of polyacrylatesis always relatively poor.

UV crosslinking requires less elaborate apparatus and is therefore ofadvantage for industrial application. In this case, free-radicalintermediates are formed which react with one another and lead tocrosslinking of the PSAs.

U.S. Pat. No. 4,717,605 describes a method of adhesively bonding opticalglass components. It does so using radiation-curable adhesives based onionically polymerizable epoxy systems and ionic photoinitiators based ontriarylsulfonium complex salts. These adhesives still contain at leastone ethylenically unsaturated compound, which can be polymerized by freeradicals in the presence of a free-radical photoinitiator.

WO 88/02879 uses a photoinitiator and an iron salt for cationicphotopolymerization. That document discloses a polymerizable compositioncomposed of a free-radically polymerizable material and a photocatalyst.The photoinitiator system is composed of π-aromatic-metal complexes ofthe form(R⁶—Fe⁺—R⁷)LZ_(k) ⁻,where R⁶ is an η⁶-aromatic, R is the anion of a cyclopentadienylcompound, L is a di- to heptavalent metal or metalloid, Z is a halogen,and k is the valence of L increased by 1. The photocatalyst systemfurther comprises a peroxidic or hydroperoxidic compound and,optionally, a metallocene.

U.S. Pat. No. 5,776,290 claims a method of preparing a coated abrasivearticle, where a first binder is present on a backing and a multiplicityof abrasive particles are present in this binder. The binder is composedof a pressure sensitive hotmelt adhesive film. This hotmelt adhesive iscured by means of an energy source so that the abrasive particles arecovered by a crosslinked coat of the adhesive. The hotmelt adhesivecrosslinked with an epoxide in this document is based on pressuresensitively adhesive polyesters.

The UV-initiated epoxide crosslinking reaction, as a highly efficientcrosslinking reaction, has not been successfully transferred to date topolyacrylate PSAs.

It is an object of the invention to provide a process for crosslinkingpolyacrylates, especially polyacrylate-based hotmelt adhesives, whichdoes not have the disadvantages of the prior art and whichadvantageously extends the state of the art.

SUMMARY OF THE INVENTION

This object is achieved by a process for crosslinking polyacrylates inaccordance with the main claim. It has been found, surprisingly andcompletely unexpectedly for the skilled worker, that polyacrylates canbe crosslinked outstandingly by UV-initiated epoxide crosslinking and interms of shear strength have advantages over conventional crosslinkingmechanisms, especially electron beam crosslinking, if they have beenfunctionalized with appropriate groups during polymerization. Thesubclaims relate to advantageous developments of the invention. A secondindependent claim relates to the use of difunctional or polyfunctionaloxygen compounds, especially difunctional or polyfunctional epoxides oralcohols, as crosslinking reagent for functionalized polyacrylates,especially functionalized acrylic hotmelt pressure sensitive adhesives.

The main claim accordingly provides a process for preparing crosslinkedpolyacrylates, in which an acrylate-based monomer mixture iscopolymerized to produce a polymer and, after the polymerization,crosslinking of the polymer is brought about by UV radiation, with up to10% by weight of copolymerizable monomers containing one or more epoxygroups and/or one or more hydroxyl groups being incorporated into thepolymers during the copolymerization and, prior to crosslinking, atleast one photocation generator and also one or more di- orpolyfunctional epoxides and/or one or more di- or polyfunctionalalcohols are added to the polymers.

DETAILED DESCRIPTION

Preferably, the free di- or polyfunctional epoxides and/or the free di-or polyfunctional alcohols are used such that one of these components ispresent in a marked excess over the other (by free epoxides or alcoholsare meant here the functionalized monomers not incorporated into thepolymer chain). In the case of an excess of free epoxides it isadvantageous if not more than 10 mol % of hydroxyl groups are present inthe form of free alcohols, based on the epoxy groups of the freeepoxides; in the case of an excess of free alcohol it is favorable tochoose the fraction of the epoxy groups in the form of free epoxides tobe not above a fraction of 10 mol %, based on the hydroxyl groups of thefree alcohol.

Where di- or polyfunctional free alcohols are present it is verypreferable not to add any di- or polyfunctional free epoxides;correspondingly, where di- or polyfunctional free epoxides are present,it is very advantageous to avoid the presence of di- or polyfunctionalfree alcohols.

In one first advantageous embodiment of the inventive process thecomposition of the monomer mixture is such that the resulting polymerspossess pressure sensitively adhering properties.

The polyacrylates to be crosslinked are preferably polyacrylates whichhave been processed and worked on prior to crosslinking in the hotmeltprocess.

Furthermore, the inventive process is very favorable if the compositionof the monomer mixture is as follows:

-   -   a) 60 to 99.5% by weight of (meth)acrylic acid and/or        (meth)acrylate esters of the following formula

-   -    where R¹=H or CH₃ and R² is an alkyl chain having 1 to 20        carbon atoms    -   b) 0.5 to 10% by weight of copolymerizable monomers which        contain one or more epoxy groups and/or one or more hydroxyl        groups,    -   c) optionally up to a maximum of 39.5% by weight of        copolymerizable olefinically unsaturated monomers containing        functional groups which activate the double bond for the        polymerization reaction,        the components of the monomer mixture adding up to 100%,        and/or if, prior to crosslinking,    -   d) 0.01 to 25% by weight of a photocation generator and    -   e) 0.1 to 5% by weight of one or more di- or polyfunctional        epoxides and/or one or more di- or polyfunctional alcohols        are added to the polymers, so that the polymers and        components d) and e) add up to 100% by weight.

The concept of the invention is to copolymerize monomers having eitherepoxy groups and/or hydroxyl groups into the polyacrylates during thepolymerization. In the presence of suitable crosslinkers, especially theabove-described di- or polyfunctional epoxides and/or di- orpolyfunctional alcohols for polyacrylates modified with epoxy groupsand/or with hydroxyl groups, it is then possible by way of thesefunctional groups to achieve crosslinking with exposure to ultravioletradiation.

In this way it is possible to use such crosslinkers for the cationiccuring or crosslinking of the functionalized polyacrylates.

In one advantageous development of the inventive process, in componenta) of the monomer mixture, the radical R² represents an alkyl chainhaving 4 to 14 carbon atoms, preferably having 4 to 9 carbon atoms.

Specific examples of such acrylic monomers, which can be used veryadvantageously, are n-butyl acrylate, n-pentyl acrylate, n-hexylacrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, and thebranched isomers thereof, such as 2-ethylhexyl acrylate, for example.

Further vinyl monomers, which can be used advantageously in combinationwith acrylate monomers in the sense of the component c), include vinylesters, vinyl ethers, vinyl halides, vinylidene halides, and nitriles ofethylenically unsaturated hydrocarbons. Specific examples are vinylacetate, N-vinylformamide, vinylpyridine, acrylamides, ethyl vinylether, vinyl chloride, vinylidene chloride, acrylonitrile, maleicanhydride, and styrene.

The monomers are preferably chosen so that the polymer prepared hasadhesive properties in accordance with the Handbook ofPressure-Sensitive Adhesives, p. 172, § 1, 1989.

For the crosslinking reaction presented, the photocation initiatorsfamiliar to the skilled worker are used, preferably one of theinitiators from the group presented below.

As photocation generators (“photoinitiators”) it is preferred to employaryldiazonium salts (“onium salts”) which can be represented generallyby the formula Ar—N=N⁺ LX⁻, LX⁻ being an adduct of a Lewis acid L and aLewis base X⁻. Particularly advantageous for LX⁻ are BF₄ ⁻, SbF₅ ⁻, AsF₅⁻, PF₅ ⁻, SO₃CF₂ ⁻. Under the influence of UV radiation there is a rapidcleavage of the molecule into the aryl halide (ArX), nitrogen, and thecorresponding Lewis acid.

Additionally, aryliodonium salts (C₆H₅)RI⁺ LX⁻, where R is an organicradical, especially diaryliodonium salts (C₆H₅)₂I⁺ LX⁻, and alsotriarylsulfonium salts (C₆H₅)₃S⁺ LX⁻, are known for use as cationicphotoinitiators. In the presence of proton donors these salts formstrong (Brönstedt) acids, which are likewise highly suitable for theinitiation of cationic polymerizations and for the inventive process.

Sulfonium salts as cation photoinitiators are present, for example, alsoin the form of the compounds H₅C₆—CO—CH₂—S⁺ LX⁻ or H₅C₆—CO—CH₂-Pyr⁺ LX⁻,with Pyr representing a nitrogen-containing heteroaromatic system (e.g.,pyridine, pyrimidine).

In one very advantageous embodiment of the inventive process thephotocation generator chosen is a triarylsulfonium hexafluoro salt fromgroup 15 of the periodic system, preferably such that the element fromgroup 15 is present in the IV oxidation state. Use is made veryfavorably of triarylsulfonium hexafluorophosphate and/ortriarylsulfonium hexafluoroantimonate.

In one very advantageous development of the inventive process astructuring of the crosslinked polyacrylates is achieved in the courseof crosslinking by covering the polyacrylates to be crosslinked with amask having regions of different UV transparency. In this case,irradiation is carried out with ultraviolet light such that certainregions of the polymer mixture are subject to different intensities ofradiation. The structuring of the polyacrylates consists in regions ofhigh crosslinking being present alongside regions of low crosslinkingand/or noncrosslinked regions within the polyacrylates.

Also claimed is the use of di- or polyfunctional oxygen compounds,especially di- or polyfunctional epoxides or alcohols, as crosslinkingreagents for the crosslinking reaction, brought about by ultravioletradiation in the presence of a photocation generator, of polyacrylatesfunctionalized by epoxy groups and/or by hydroxyl groups.

The basic principles of the invention are set out below. The polymersfor crosslinking are prepared from the monomer mixture by free-radicalpolymerization such that their molecular weight lies within the order ofmagnitude of 250,000–1,000,000 g/mol.

The free-radical polymerization may be conducted in the presence of oneor more organic solvents and/or in the presence of water or without anysolvent. It is preferred to use as little solvent as possible. Dependingon conversion rate and temperature, the polymerization time is between 6and 48 h.

In the case of solution polymerization, solvents used are preferablyesters of saturated carboxylic acids (such as ethyl acetate), aliphatichydrocarbons (such as n-hexane or n-heptane), ketones (such as acetoneor methyl ethyl ketone), special boiling point spirit, or mixtures ofthese solvents. As polymerization initiators use is made of customaryfree-radical-forming compounds, such as peroxides and azo compounds, forexample. Initiator mixtures as well can be used. In the polymerizationit is further possible to use thiols as regulators for lowering themolecular weight and reducing the polydispersity. As what are known aspolymerization regulators it is possible to use, for example, alcoholsand ethers.

In one very favorable procedure, after the polymerization, the PSA iscoated from solution onto a backing material. In one very preferredvariant the polymerization medium is removed under reduced pressure,this operation being carried out at elevated temperatures, in the rangefrom 80 to 150° C., for example. The polymers can then be used andcoated in the solvent-free state, in particular as hotmelt PSAs. Forselected applications it is also of advantage to prepare and process thepolymers without any solvent.

The blends described in this invention can be modified further in orderto achieve the optimum technical adhesive properties.

By way of example, the polymers for preparing PSAs are optionallyblended with one or more resins. Examples of resins which can be usedinclude terpene resins, terpene-phenolic resins, C₅ and C₉ hydrocarbonresins, pinene resins, indene resins, and rosins, alone and also incombination with one another, and also their disproportionated,hydrogenated, polymerized, and esterified derivatives and salts. Inprinciple, however, it is possible to use any resins which are solublein the corresponding polyacrylate; reference may be made in particularto all aliphatic, aromatic, and alkylaromatic hydrocarbon resins,hydrocarbon resins based on pure monomers, hydrogenated hydrocarbonresins, functional hydrocarbon resins, and natural resins.

Furthermore, it is possible to add various fillers (for example, carbonblack, TiO₂, solid or hollow beads of glass or other materials,nucleators), blowing agents, compounding agents and/or aging inhibitors.

In one particular advantageous development, plasticizers are admixed inorder to improve the flow behavior of the PSA.

A development which makes the process of the invention particularlyadvantageous for the preparation of, for example, adhesive tapes isdistinguished by the further processing of the PSA from the melt.

As backing material, for adhesive tapes for example, it is possible touse the materials which are customary and familiar to the skilledworker, such as films (polyester, PET, PE, PP, BOPP, PVC), nonwovens,foams, wovens and woven films, and also release paper (glassine, HDPE,LDPE).

The inventive crosslinking of the polyacrylates or of the (hotmelt) PSAstakes place by brief UV irradiation in the range from 200 to 400 nmusing commercially customary high-pressure or medium-pressure mercurylamps with an output of, for example, 80 to 200 W/cm. It may beappropriate to adapt the lamp output to the belt speed or to run thebelt slowly while shading it off partly in order to reduce the thermalload thereon. The irradiation time is guided by the construction andoutput of the respective lamps.

The inventive process can be utilized outstandingly to prepare structurepolyacrylates, especially structured (hotmelt) PSAs. One process forpreparing structured polyacrylates by structured crosslinking ofpolyacrylate mixtures is distinguished in that the base polymer mixtureis irradiated with ultraviolet light in such a way that only certainregions of the polymer mixture are exposed to the UV radiation.

The process for preparing structured polyacrylates may be conducted inparticular such that the base polymer mixture is irradiated withultraviolet light through a perforated mask in such a way that onlycertain regions of the polymer mixture are exposed to the UV radiation.

Alternatively, the structuring of the polymer mixture for curing may beachieved by using, rather than the perforated mask, a film whosetwo-dimensional extent has regions of different UV transparency, so thatcertain regions of the polymer mixture are exposed to differentintensities of the UV radiation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the principle of selective irradiation

In FIG. 1, the principle of selective irradiation is illustrated by adiagram. In the figure, the irradiation of the acrylate composition (2)through a perforated mask (1) is shown, the acrylate composition (2)being present on the backing (3). In accordance with the main claim, theacrylate composition (1) is admixed with a photocation initiator whichinitiates the crosslinking reaction as a result of UV light (4). Theultraviolet rays (4) are able to penetrate the mask (1) only in theregion of the perforations (11), so that after irradiation the situationdepicted in the bottom part of the diagram results: the pressuresensitive adhesive (2) has hard segments of high crosslinking (21)alongside noncrosslinked segments (22).

The polymer chains at the margins of the hard regions extend into thesoft regions; accordingly, the hard regions, which are inherently ofhigh viscosity, are linked with the soft regions and so hinder theseregions in their mobility, so that the structural strength of theadhesive is increased. Moreover, these hard segments increase thecohesion of the pressure sensitive adhesive. In contrast, the softsegments facilitate the flow of the adhesive on the substrate and soincrease the bond strength and the tack. A great influence on thetechnical adhesive properties is exerted by the percentage fraction ofthe irradiated surface area, and also by the size of the segmentsgenerated.

EXAMPLES

The following exemplary experiments are intended to illustrate thecontent of the invention, without wishing to restrict the inventionunnecessarily through the choice of the examples.

Test Methods

The polyacrylate compositions and their crosslinked products werecharacterized using the test methods described below:

Shear Strength (Test A1, A2)

A 13 mm wide strip of the adhesive tape was applied to a smooth andcleaned steel surface. The application area was 20 mm×13 mm(length×width). The subsequent procedure was as follows:

Test A1: At room temperature, a 1 kg weight was fastened to the adhesivetape and the time which elapsed until the weight fell off was measured.

Test A2: At 70° C., a 1 kg weight was fastened to the adhesive tape andthe time which elapsed until the weight fell off was measured.

The measured shear stability times are each reported in minutes, andcorrespond to the average of three measurements.

180° Bond Strength Test (Test B)

A 20 mm wide strip of an acrylic PSA coated onto a polyester was appliedto steel plates. The PSA strip was pressed onto the substrate twiceusing a 2 kg weight. The adhesive tape was then peeled immediately fromthe substrate at 300 mm/min and at an angle of 180°. The steel plateswere washed twice with acetone and once with isopropanol. For themeasurements on the PE substrate, only new plates were used. The resultsare reported in N/cm and are averaged from three measurements. Allmeasurements were made at room temperature under climatized conditions.

Determination of the Gel Fraction (Test C)

The carefully dried, solvent-free adhesive samples are welded into anonwoven polyethylene (Tyvek) pouch. The gel value (weight fraction ofthe polymer that is not soluble in toluene) is determined from thedifference in the sample weights before and after extraction withtoluene.

Samples Investigated

The samples used for the experiments were prepared as follows:

The polymers were prepared conventionally by free-radicalpolymerization; the average molecular weight is approximately 800,000g/mol.

Example 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of glycidyl methacrylate, 196 g of n-butyl acrylate,196 g of 2-ethylhexyl acrylate and 266 g of acetone/isopropanol (97:3).After nitrogen gas had been passed through the reaction solution withstirring for 45 minutes, the reactor was heated to 58° C. and 0.4 g ofAIBN [2,2′-azobis(2-methylbutyronitrile)] was added. Subsequently, theexternal heating bath was heated to 75° C. and the reaction wasconducted constantly at this external temperature. After 4 and 6 h,dilution was carried out with in each case 150 g of acetone/isopropanolmixture (97:3). After a reaction time of 48 h, the polymerization wasterminated and the reaction vessel was cooled to room temperature.

The product was subsequently diluted with 150 g of acetone, and 12.8 gof bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate (50%strength solution in propylene carbonate; Cyracure UVI-6994® [UNIONCARBIDE]) were added. The mixture was coated from solution at 50 g/m²onto an isocyanate-primed PET film and heated at 120° C. for 10 minutes.UV irradiation was carried out using a Xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m². After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 2

The procedure of Example 1 was repeated. The polymer was subsequentlydiluted with 150 g of acetone, and 12.8 g of bisphenol A and 8 g oftriarylsulfonium hexafluorophosphate (50% strength solution in propylenecarbonate; Cyracure UVI-6990® [UNION CARBIDE]) were added. The mixturewas coated from solution at 50 g/m² onto an isocyanate-primed PET filmand heated at 120° C. for 10 minutes. UV irradiation was carried outusing a xenon chloride lamp (VIB 308 bulb [FUSION]) with a radiationintensity of 160 W/m². After one lamp pass with a belt speed of 20m/min, the adhesive tape specimens were further heated for 10 minutesand then tested in accordance with test methods A to C.

Example 3

The procedure of Example 1 was repeated. The polymer was coated at 50g/m² onto an isocyanate-primed PET film, dried at 120° C. for 10minutes, cured with electron beams (230 kV acceleration voltage, EBCunit from Crosslinking), and then subjected to technical adhesivetesting using test methods A to C.

Example 4

The procedure of Example 1 was repeated. 4 g of triarylsulfoniumhexafluoroantimonate (50% strength solution in propylene carbonate;Cyracure UVI-6994® [UNION CARBIDE]) were added to the blend.

Example 5

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of glycidyl methacrylate, 4 g of acrylic acid, 194 g ofn-butyl acrylate, 194 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

The product was subsequently diluted with 150 g of acetone, and 12.8 gof bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate (50%strength solution in propylene carbonate; Cyracure UVI-6994® ([UNIONCARBIDE]) were added. The mixture was coated from solution at 50 g/m²onto an isocyanate-primed PET film and heated at 120° C. for 10 minutes.UV irradiation was carried out using a xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m². After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 6

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of glycidyl methacrylate, 4 g of acrylic acid, 20 g ofmethyl acrylate, 20 g of N-tertbutylacrylamide, 348 g of 2-ethylhexylacrylate and 266 g of acetone/isopropanol (97:5). After nitrogen gas hadbeen passed through the reaction solution with stirring for 45 minutes,the reactor was heated to 58° C. and 0.4 g of AIBN[2,2′-azobis(2-methylbutyronitrile)] was added. Subsequently, theexternal heating bath was heated to 75° C. and the reaction wasconducted constantly at this external temperature. After 4 and 6 h,dilution was carried out with in each case 150 g of acetone/isopropanolmixture (97:5). After a reaction time of 48 h, the polymerization wasterminated and the reaction vessel was cooled to room temperature. Theaverage molecular weight is approximately 710,000 g/mol.

The product was subsequently diluted with 150 g of acetone, and 12.8 gof bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate (50%strength solution in propylene carbonate; Cyracure UVI-6994 (D [UNIONCARBIDE]) were added. The mixture was coated from solution at 50 g/m²onto an isocyanate-primed PET film and heated at 120° C. for 10 minutes.UV irradiation was carried out using a xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m². After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 7

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of hydroxyethyl methacrylate, 196 g of n-butylacrylate, 196 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

The product was subsequently diluted with 150 g of acetone, and 20 g ofbisepoxidized bisphenol A Rütapox 164™ (from Bakelite AG) and 8 g oftriarylsulfonium hexafluoroantimonate (50% strength solution inpropylene carbonate; Cyracure UVI-6994® ([UNION CARBIDE]) were added.The mixture was coated from solution at 50 g/m² onto anisocyanate-primed PET film and heated at 120° C. for 10 minutes. UVirradiation was carried out using a xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m . After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 8

The procedure of Example 7 was repeated. The polymer was coated at 50g/m² onto an isocyanate-primed PET film, dried at 120° C. for 10minutes, cured with electron beams (230 kV acceleration voltage, EBCunit from Crosslinking), and then subjected to technical adhesivetesting using test methods A–C.

Example 9

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of hydroxyethyl methacrylate, 4 g of acrylic acid, 194g of n-butyl acrylate, 194 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

The product was subsequently diluted with 150 g of acetone, and 20 g ofbisepoxidized bisphenol A Rütapox 164™ (from Bakelite AG) and 8 g oftriarylsulfonium hexafluoroantimonate (50% strength solution inpropylene carbonate; Cyracure UVI-6994® [UNION CARBIDE]) were added. Themixture was coated from solution at 50 g/m² onto an isocyanate-primedPET film and heated at 120° C. for 10 minutes. UV irradiation wascarried out using a xenon chloride lamp (VIB 308 bulb [FUSION]) with aradiation intensity of 160 W/m². After one lamp pass with a belt speedof 20 m/min, the adhesive tape specimens were further heated for 10minutes and then tested in accordance with test methods A to C.

Example 10

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 8 g of hydroxyethyl methacrylate, 4 g of acrylic acid, 20 gof methyl acrylate, 20 g of N-tert-butylacrylamide, 348 g of2-ethylhexyl acrylate and 266 g of acetone/isopropanol (97:5). Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.4 g of AIBN[2,2′-azobis(2-methylbutyronitrile)] was added. Subsequently, theexternal heating bath was heated to 75° C. and the reaction wasconducted constantly at this external temperature. After 4 and 6 h,dilution was carried out with in each case 150 g of acetone/isopropanolmixture (97:5). After a reaction time of 48 h, the polymerization wasterminated and the reaction vessel was cooled to room temperature. Theaverage molecular weight is approximately 686,000 g/mol.

The product was subsequently diluted with 150 g of acetone, and 20 g ofbisepoxide bisphenol A Rütapox 164™ (from Bakelite AG) and 8 g oftriarylsulfonium hexafluorophosphate (50% strength solution in propylenecarbonate; Cyracure UVI-6990® [UNION CARBIDE]) were added. The mixturewas freed from the solvent under reduced pressure and then coated at 50g/m² from the melt at 120° C. through a slot die onto anisocyanate-primed PET film. UV irradiation was carried out using a Xenonchloride lamp (VIB 308 bulb [FUSION]) with a radiation intensity of 160W/m². After one lamp pass with a belt speed of 20 m/min, the adhesivetape specimens were further heated for 10 minutes and then tested inaccordance with test methods A to C.

Results

The results of the technical adhesive tests of Examples 1 to 3 are shownin Table 1.

TABLE 1 SST 10 N, SST 10 N, 70° C. BS steel Gel value [min] [min] [N/cm][%] Example (test A1) (test A2) (test B) (test C) 1 +10000 7885 3.8 70 2+10000 +10000 3.7 65  3^(a) 255 30 4.0 43  3^(b) 1085 125 4.0 63^(a)irradiated with an EB dose of 40 kGy; ^(b)irradiated with an EB doseof 60 kGy SST: Shear stability times BS: Bond strength

The investigations of Examples 1 and 2 demonstrate that with the processof the invention it is possible to prepare pressure sensitive adhesiveshaving a very high shear strength. Both triarylsulfoniumhexafluorophosphate and triarylsulfonium hexafluoroantimonate aresuitable photoinitiators for epoxy crosslinking. The epoxy functions areincorporated by polymerization in the polymer chain by means of glycidylmethacrylate. For crosslinking, bisphenol A is used. Example 3demonstrates the superiority of the crosslinking of the invention overthe conventional electron beam curing. With approximately the same gelvalue, the shear strength of the electron-beam-crosslinked samples liesbelow that of Examples 1 and 2.

The results of the technical adhesive evaluations of Examples 4 to 6 areshown in Table 2.

TABLE 2 SST 10 N, SST 10 N, 70° C. BS steel Gel value [min] [min] [N/cm][%] Example (test A1) (test A2) (test B) (test C) 4 +10000 +10000 3.6 695 +10000 +10000 3.6 73 6 +10000 +10000 3.5 70 SST: Shear stability timesBS: Bond strength

Examples 4 to 6 underline the universal usefulness of the crosslinkingprocess of the invention for acrylic PSA. Thus, for example, it is evenpossible to use acrylic acid as a comonomer and to carry out epoxycrosslinking. Example 4 is evidence that even the photoinitiatorfraction can be reduced further for efficient crosslinking. The shearstrength is fully retained. It is also possible to lower the averagemolecular weight. Example 6 possesses an average molecular weight ofapproximately 700,000 g/mol and yet achieves the optimum shear strengthas a result of the epoxy crosslinking of the invention.

The results of Examples 7 to 10 are summarized in Table 3 below.

TABLE 3 SST 10 N, SST 10 N, 70° C. BS steel Gel value [min] [min] [N/cm][%] Example (test A1) (test A2) (test B) (test C) 7 +10000 6590 3.8 67 8^(a) 345 55 4.1 39  8^(b) 1285 160 4.2 59 9 +10000 +10000 3.6 71 10 +10000 +10000 3.7 68 ^(a)irradiated with an EB dose of 40 kGy;^(b)irradiated with an EB dose of 60 kGy SST: Shear stability times BS:Bond strength

For Examples 7, 9 and 10, the converse route was taken. Bycopolymerization of the hydroxyethyl methacrylate (HEMA), hydroxylgroups were incorporated randomly along the polymer chain and were thencrosslinked using a difunctional epoxide with acid catalysis. Via thisroute as well, the crosslinking reaction of the invention proceeds withsignificantly greater efficiency than the electron beam curingcomparison which was conducted. Moreover, here again, differentcomonomer compositions are tolerated. Example 10 emphasizes thatcompositions with a relatively high degree of regulation can beprocessed as hotmelts and, with the crosslinking, likewise extend intothe high shear strength range.

A variant of the process which has been found particularly efficient isthat wherein the epoxy-functionalized polyacrylates were reacted withhydroxy-functionalized crosslinkers (alcohols).

1. A process for preparing crosslinked polyacrylates, in which a monomermixture comprising (meth)acrylates is copolymerized to produce a polymerand, after the polymerization, the polymer is subjected to a hotmeltprocess after which crosslinking of the polymer is brought about byultraviolet radiation, wherein 0.5 to 10% by weight of the monomermixture of copolymerizable monomers containing one or more epoxy groupsare incorporated into the polymers during the copolymerization byglycidyl methacrylate, and, prior to crosslinking, at least onephotocation generator and also one or more di- or polyfunctionalepoxides and/or alcohols are added.
 2. The process of claim 1, whereinthe monomer mixture is one which will result in polymers which arepressure sensitively adhesive.
 3. The process of claim 1, wherein thecomposition of the monomer mixture is as follows: a) 60 to 99.5% byweight of (meth)acrylic acid and/or (meth)acrylate esters of thefollowing formula

 where R¹=H or CH₃ and R² is an alkyl chain having 1 to 20 carbon atoms,b) 0.5 to 10% by weight of copolymerizable monomers which contain one ormore epoxy groups and/or one or more hydroxyl groups, c) optionally upto a maximum of 39.5% by weight of copalymerizable olefinicallyunsaturated monomers containing functional groups which activate thedouble bond for the polymerization reaction, the components of themonomer mixture adding up to 100%, and/or wherein, prior tocrosslinking, d) 0.01 to 25% by weight of a photocation generator and e)0.1 to 5% by weight of glycidyl methacrylate is added to the polymers,so that the polymers and components d) and e) add up to 100% by weight.4. The process of claim 1, wherein a triarylsulfonium hexafluoro saltfrom group 15 of the periodic system is used as photocation generator.5. The process of claim 1, wherein, in the course of crosslinking,structuring of the crosslinked polyacrylates is achieved by covering thepolyacrylates to be crosslinked with a mask having regions of differentultraviolet transparency, the structuring of the polyacrylates being thepresence within the polyacrylates of regions of high crosslinkingalongside regions of low crosslinking and/or noncrosslinked regions. 6.A method of crosslinking by ultraviolet radiation in the presence of aphotocation generator, of polyacrylates functionalized by epoxy groups,which comprises carrying out said crosslinking with di- orpolyfunctional epoxides or alcohols as crosslinking reagents.
 7. Theprocess of claim 3, wherein R₂ is an alkyl chain having from 4 to 14carbon atoms.
 8. The process of claim 3, wherein R₂ is an alkyl chainhaving from 4 to 9 carbon atoms.
 9. The process of claim 4, wherein saidtriarylsulfonium hexafluoro salt is selected from the group consistingof triarylsulfonium hexafluorophosphate, triarylsulfoniumhexafluoroantimonate and mixtures thereof.